1. Field of the Invention
The present invention relates to data communications over a telephone network, and more particularly to a network interface device, method, and article of manufacture providing for high bit rate data communication over robbed bit trunks.
2. Description of the Prior Art
Recently V.pcm modem technology has emerged which uses pulse amplitude modulation (PAM) techniques to transfer data at 56,000 bits per second. This technology is based on the premise that the 56K transmitter is digitally connected to the public telephone network and can control the pattern sent to the telephone network codec (digital to analog converter) driving the analog link to the receiver.
The digital portion of the telephone network transfers data as 8,000 8-bit samples per second, or 64,000 bits per second per channel (i.e., one channel of a T1 trunk using a DS-1 structure). However, due to a variety of factors, prior art transfer rates are limited to 56,000 bps. This leaves 8,000 bits per second capacity that does not get utilized for communication.
There are two main reasons why current technology is limited to 56,000 bits per second. The first concerns granularity and discrimination. These 56K modems use Pulse Amplitude Modulation (PAM) for transmission with eight bits per sample. Current signal processing technology cannot resolve the least-significant bit in the recovered signal, so only 128 different codes are sent, out of 256 possible codes, lowering the maximum bit rate to 56,000 from 64,000 bits per second. This problem is particularly difficult with codes corresponding to low signal amplitudes, since telephony codecs use a logarithmic (.mu.-law) characteristic. For these codes, not only are the absolute levels lower, but discrete levels are closer together, compared to higher amplitudes. This problem does not apply to the ISDN technology where no telephony codec is involved.
The second reason why current technology is limited to 56,000 bps is so-called "robbed-bit" signaling. On certain trunks, the digital portion of North American telephone systems `robs` the least-significant bit (LSB) of every sixth sample, replacing it with signaling information. Signaling generally means the passing of call related information such as hookswitch status, address, and number being called. This signaling information does not change during the useful portion of a telephone call and thus the robbed-bit signaling results in having the bit stuck or set to one value, and hence correct half the time. This is unnoticed in voice telephone conversations, and only marginally affects conventional (QAM, DPSK, FSK, etc.) modems, but irrevocably corrupts PAM transmissions. With robbed bit signaling even though only one bit out of every six samples is used for signaling, there is no way to communicate between ends which bit is corrupt, so all LSBs must be unused.
Robbed-bit signaling is also a problem for ISDN links where both ends of a connection are digitally connected. ISDN devices coupled through a robbed-bit trunk can only transfer data at 56,000 bits per second, avoiding all use of the LSB, even though only one bit out of six samples is corrupt. Therefore, maximum throughput for ISDN devices coupled through a robbed-bit link is 56,000 bits per second. Thus, robbed-bit signaling limits throughput on ISDN systems using a robbed-bit trunk.
These unresolved problems and deficiencies are clearly felt in the art and are solved by the invention in the manner described below.